Fuel cells have been proposed as a power source for electric vehicles and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called “membrane-electrode-assembly” comprising a thin, solid polymer membrane-electrolyte having an anode on one face of the membrane-electrolyte and a cathode on the opposite face of the membrane-electrolyte. The anode and cathode typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material intermingled with the catalytic and carbon particles.
The membrane-electrode-assembly is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, and may contain appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (i.e., H2 & O2/air) over the surfaces of the respective anode and cathode.
Bipolar PEM fuel cells comprise a plurality of the membrane-electrode-assemblies stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate or septum. The septum or bipolar plate has two working faces, one confronting the anode of one cell and the other confronting the cathode on the next adjacent cell in the stack, and bipolar plate electrically conducts current between the adjacent cells. Contact elements at the ends of the stack contact only the end cells and are referred to as end plates.
In an H2—O2/air PEM fuel cell environment, the bipolar plates and other contact elements (e.g. end plates) are in constant contact with highly acidic solutions (pH 3–5) containing F−, SO4−, SO3−, HSO4−, CO3−, and HCO3−, etc. Moreover, the cathode operates in a highly oxidizing environment, being polarized to a maximum of about +1 V (vs. the normal hydrogen electrode) while being exposed to pressurized air. Finally, the anode is constantly exposed to super atmospheric hydrogen. Hence, contact elements made from metal must be resistant to acids, oxidation, and hydrogen embrittlement in the fuel cell environment while maintaining good electrical conductivity. As few metals exist that meet this criteria, contact elements have often been fabricated from large pieces of graphite, which is corrosion-resistant, and electrically conductive in the PEM fuel cell environment. However, graphite is quite fragile, and quite porous making it extremely difficult to make thin gas impervious plates therefrom.
Lightweight metals, such as aluminum and its alloys, have also been proposed for use in making fuel cell contact elements. Such metals are more conductive than graphite, and can be formed into very thin plates, which increases the gravimetric efficiency of the fuel cell. Unfortunately, such light weight metals are susceptible to corrosion in the hostile PEM fuel cell environment, and contact elements made therefrom potentially dissolve (e.g., in the case of aluminum) in the fuel cell environment. Other desirable metals include stainless steel or titanium, which while more corrosion resistant, are susceptible to passivation, where metal oxides form at the surface. The metal oxides increase surface contact resistance, making them undesirable to use directly in a fuel cell current collector due to impermissibly high electrical resistance.
In light of the corrosion sensitivity of lightweight metals, such as aluminum, efforts have been made to develop protective coatings. However, some of these protective methods increase the electrical resistance of the aluminum plate to unacceptable levels. Other methods of protection keep the conductivity at an acceptable level, but do not sufficiently achieve the desired level of protection, or are cost prohibitive. Accordingly, there is a need for conductive bipolar plate which is corrosion resistant and electrically conductive to promote power output and efficiency of the fuel cell.